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[Openvpn-users] Problem with openvpn on multi-homed host


  • Subject: [Openvpn-users] Problem with openvpn on multi-homed host
  • From: Evan Harris <eharris@xxxxxxxxxxxxx>
  • Date: Fri, 5 Mar 2004 19:53:19 -0600 (CST)

I have an openvpn server that is multihomed.  It has a private ip on the
eth0 interface, and an external public ip on eth1.

I'm trying to setup laptops so there is no configuration requred wether they
are inside or outside the network, openvpn just establishes a connection and
works.

So, I've configured the openvpn clients to always establish the tunnel to
the public ip address of the server.  It should work, since the default
route inside the network will still be able to get there.

The problem comes up from the fact that openvpn doesn't appear to reply to
the other end of the tunnel with udp packets having the same source address
as the destination of the original packets.  Its sent replies have a source
ip address of the interface the reply packets were sent via.

Normally that would be ok, because you can use the --float option and when
the replies from the server come back with a different ip, the peer (the
notebook) will see it and change the tunnel destination address to the
source of the packets, in this case the private ip of the server.

Now here's the twist that causes the problem: we have a multilocation
internal network using frame relay.  Many of the remote offices use a
wireless network internally, and many of the notebooks attach using those
links.

Because of the security issues of the wireless networks, the wireless
routers are firewalled so they can't talk to internal machines directly, so
the only traffic allowed to pass from those routers are packets destined for
the public ip of the tunnel server.  So we don't want to use the --float
option.

I just want openvpn to reply with the same ip!  Can it be forced to?

Tested with openvpn 1.5.0.

Evan



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